1. Field of the Invention
The present invention relates to a surface treatment method for surface-treating a metal member molded by casting or expanding metal, to thereby produce a uniform surface at a lower cost; to metal goods that are surface-treated to form thereon an anticorrosive surface treatment coating; and to metal goods wherein a corrosion-resistant paint film is formed on that surface treatment coating by the application of corrosion-resistant paint thereon.
2. Discussion of the Background
Many metals are susceptible to corrosion at high temperature and humidity, particularly in a salt-contained atmosphere, if not coated with paint. When coated with paint, a molded metal member must be subjected to a pre-cleaning process to clean a mold release agent, commonly used for molding the metal, remaining on a surface of the molded metal member. If the molded metal member is coated with paint without being subjected to the pre-cleaning, the adhesion of the paint film to the metal member will be significantly reduced, triggering early corrosion.
Especially for a molded metal member having crimples and cracks on its surface, the mold release agent intrudes in between the crimps and cracks, so that the mold release agent remains therein easily even when the molded metal member is subjected to the pre-cleaning process.
To address these problems, the following three processes are commonly used. First, (a) a pre-cleaning process using alkali degreasing, acid pickling or blast polishing is performed to clean the surface of the metal member. Then, (b) a base coat process is performed to subject the surface of the metal member to a chemical conversion treatment using chromate and the like. Thereafter, (c) a paint application process is performed to apply a corrosion-resistant paint to the surface of the treated metal member.
However, in (a), the blast polishing of the pre-cleaning process has the disadvantage that it is difficult to clean deep concaves of a molded product having a complicated shape, and the pre-cleaning process using alkali degreasing or acid pickling has the disadvantage of causing corrosion easily when contacted with remaining water. In (b), the chromate treatment has the disadvantage of producing ill effects on the human body. In (c), the paint application process has the disadvantage of failing to protect against corrosion during the time between the base coat process and the paint application process. Effective alternative chemical conversion treatments have not yet been proposed.
In recent years, various metal materials, including lightweight alloy materials typified by magnesium alloy, have been used as environmentally suitable materials to produce molded products in many fields. Many of the molded products have a complicated shape and are covered with a protective corrosion-resistant outer layer. However, because existing surface treatment methods do not provide the outer layer with satisfactory adhesion, the molded products do not have satisfactory corrosion resistance. Development of a novel surface treatment method that is suitable for these metal members and that can also provide an excellent corrosion proof coating and development of a novel corrosion-resistant agent suitably used in the method are now being awaited. The creation of such a novel surface treatment method and the corrosion-resistant agent could provide the metal members with expanded applications.
The present inventors have previously proposed a surface treatment method in which a cast product is surface treated by heating under pressure in a liquid (Japanese Patent Application No. 2001-126623). This method can eliminate the need for the pre-cleaning process; enables even a cast product having a complicated form to be properly surface-treated; and besides can make the surface-treated surface uniform, thus producing a cast product having good corrosion resistance.
However, when an acid surface-treatment liquid is used, a molded metal product, including a cast product, can sometimes be corroded by the acid, depending on the kind of metal member and the kind of acid. This corrosion can cause a reduction in dimensions and pitting corrosion to produce a non-uniform surface. On the other hand, when an alkaline compound is added to the surface-treatment liquid, in order to try to prevent these problems, the compound is precipitated to deteriorate the surface treatment liquid, producing the problem that the surface treatment coating cannot be formed stably.